This invention relates to electrodeless discharges and, more particularly, to improving the conversion efficiency of electrical power to light by means of an electrodeless discharge.
The incandescent lamp is a principal source of lighting in homes and businesses. However, its light emitting filament evaporates and becomes weak with use, hence is easily fractured or dislodged from its supports. Thus, the lifetime of an incandescent lamp is short and unpredictable. More importantly, the efficiency of an incandescent lamp in converting electrical power to light is very low, e.g., approximately 15 lumens of light per watt of electrical power.
Fluorescent lamps are generally more efficient and durable than incandescent lamps. However, the conventional fluorescent lamp requires both a ballast supply and a special fixture which must be installed before a lamp can be used. Further, the electrodes are subject to disintegration over a period of time.
Alternative concepts exist within the prior art which provide for lighting devices in which electrodes are absent. No such devices are yet, however, commercially available.
One class of device is described by J. M. Anderson in U.S. Pat. No's 3,500,118 and 3,521,120, and is based on the concept that air-cored high frequency transformers, when employed to transfer power into electrodeless arc discharges, yield inefficient coupling of energy to the discharge, resulting in a loss of power by radiation which is prohibitive, and which may be dangerous. In this respect, it has been stated that such devices have never been successfully operated for useful periods at any reasonable efficiency. Thus, a claim is made that a ferrite core within the induction coil is required to promote efficient coupling of power to an electrodeless arc.
Ferrite materials, when employed in such a application, promote considerable inefficiency. Initially, such materials increase the inductance of the induction coil to such an extent that adequately high frequency operation is impossible. Thus, ferrite-cored devices are inherently low frequency devices, and their operation requires excessively high induction field strengths. Secondly, hysteresis losses in ferrite cores promote heating of the ferrite material, and the circulating current in the coil wound about the ferrite core also promotes conductive heating of the core, as does direct and indirect optical radiation emitted by the discharge and falling onto the core or intervening material, and as do any ohmic effects present in the core. It is well known that at the Curie temperature, usually in the vicinity of 100.degree. C - 150.degree. C, the permeability of most common ferrites reduces discontinuously to a very low value. When this core temperature is achieved during system operation, the induction coil is effectively unloaded by the large decrease in its inductance, the magnitude of the induction field decreases substantially, the discharge extinguishes, and the now unloaded transistor rapidly fails due to thermal runaway. Also, the cost of a suitable ferrite core for such devices is of the same order as the cost of the entire remainder of the electronic system powering the discharge. Thus, not only does such a core promote system inefficiency and reduce its operational reliability, but it substantially raises the cost of the electronic system a well.
In the referenced prior art, an auxiliary discharge starting circuit impresses an electric field on the lamp to provide initial ionization. Subsequently, energy is coupled into the discharge by means of the alternating magnetic field of the ferrite core. Input voltage and current to the five turn primary winding of the ferrite transformer, respectively, are 50 volts and 0.6 amperes at a frequency of 50 kilohertz, while the induced voltage and current are given as approximately 10 volts and 3 amperes, respectively, with a core loss of approximately 3 watts. The luminous efficacy claimed for the device is 40 lumens per watt, but substantial circuit losses are not included in this figure.
Laboratory studies have confirmed that the rectified input voltage to this circuit, when drawn from a 110 VAC line, is 155 volts. A 50 volt input to the ferrite transformer is obtainable only under class A conditions where a 30 watt high frequency output requires circuit input power in excess of 60 watts. Therefore, overall circuit luminous efficacy for this device can be no greater than 20 lumens per watt, a figure only slightly in excess of the luminous efficacy of a conventional incandescent lamp.